The present invention can be more easily understood with respect to a light source for use in backlit LCD displays. Such displays are widely used in image display applications including computer displays and stand alone displays of the type used in advertising and the like. These displays utilize a pixilated image plane in which each pixel includes a color filter and an LCD shutter. The pixels are illuminated with white light from a source behind the image plane. A viewer in front of the image plane sees an image that is generated by opening the individual pixel shutters for a time proportional to the intensity of light that is to be perceived as originating from that pixel. A color image is generated by utilizing three closely spaced pixels having different color filters. The image is repeatedly generated to give the impression of a still or moving picture with the desired color intensity values at each pixel.
Ideally, the light source used for the display is a white light source having a color temperature that can be set for various applications. At present, most displays of this type utilize a fluorescent light source. Fluorescent light sources have significantly longer life-times and provide more light output for a given electrical power into the light source than incandescent sources. Unfortunately, the color temperature of such sources is not easily varied, as the output spectrum depends mainly on the phosphors used in the device.
In principle, white light sources based on light-emitting diodes (LEDs) can provide the desired color temperature variation and life-times. A light source having three different color LEDs can provide a wide range of colors, and hence, duplicate a wide range of color temperatures. Unfortunately, with the exception of red LEDs, currently available LEDs have a number of drawbacks that inhibit their use in such backlit displays.
First, the light available from a single LED is often too low to provide the necessary brightness. As a result, a large number of LEDs of each color are needed. The cost of these LEDs is too high to make such sources competitive with fluorescent displays. In addition, the output of each LED changes with age, drive current, and temperature. As a result, some form of feedback system is typically used to continuously adjust the drive current or duty factor of each LED to compensate for drift in the output from these and other factors. Furthermore, LEDs that emit colors other than red have lower power conversion efficiencies than the red LEDs. These LEDs also tend to be more expensive to manufacture than red LEDs.